1. Field of the Invention
The present invention relates to an electrode based on carbonaceous material, for a rechargeable electrochemical lithium generator.
2. Description of Related Art
Rechargeable electrochemical generators based on alkaline metals, such as lithium generators, have a strong advantage due to their energy density, which is far greater than that of most of the standard electrochemical pairings which operate in aqueous media. Their development, however, has been hindered by the inadequacies of the negative electrode. In effect, metallic lithium reduces the electrolyte, which decomposes during recharging. Thus, an anodic deposit of lithium is observed, which has a powdery, dendritic nature that increases the risk of a short-circuit and leads over time to a substantial loss of reversible capacity. Moreover, the formation of this deposit consumes the electrolyte in an irreversible way, which also limits the life of the generator.
In order to remedy these drawbacks, it has been proposed to use intercalated electrodes which allow the electrode-electrolyte interface to be stabilized. This is particularly the case with electrodes based on carbonaceous material, which are generally composite electrodes.
But during the first charging, the intercalation of lithium into the electrode is carried out correctly only after the passivation of its surface which results from the inevitable reduction of the electrolyte to a low potential. This passivation consumes a large part of the initial capacity of the positive electrode in an irreversible way, which leads to an irreversible loss of capacity of the generator, and consequently, to a low efficiency of the first charge and discharge cycle.
A known solution which makes it possible to avoid the problem of irreversible capacity loss is described in the patent application WO-90/13924. It consists of adding a foil of metallic lithium to the contact of the electrode based on carbonaceous material. This assembly constitutes a local cell which allows the insertion of lithium ions into the electrode as soon as the electrolyte has been introduced. The drawback of this system is that it gives the electrolyte an excessive thickness, which makes the generator difficult to assemble and reduces its energy per unit volume. On the other hand, when the lithium is displaced toward the electrode based on carbonaceous material, the excessive thickness disappears, causing a loosening of the constituents of the generator which is detrimental to its operation.
In the best cases of the prior art, the residual reversible capacity, that is the specific reversible capacity remaining after the first cycle, of the electrodes based on carbonaceous material reaches values which hardly ever exceed 300 mAh/g and the efficiency of the first cycle, that is the ratio between the residual reversible capacity at the first cycle and the initial charge capacity, remains less than 90%.
Accordingly, the object of the present invention is the production of an electrode based on carbonaceous material which has both a residual reversible capacity which is stable and distinctly higher than 300 mAh/g, and an efficiency at the first cycle which is distinctly higher than 90%.